The Earth is a sphere-like shape (like the scotch egg!) with a diameter of about 12,700 kilometres. As we go deeper and deeper into the earth the temperature and pressure rises.
The crust is on average 20 km thick. If the Earth was the size of a football, the crust would only be about ½ millimetre thick. The thinnest parts are under the oceans (Oceanic Crust) and go to a depth of roughly 10 kilometres. The thickest parts are the continents (Continental Crust) which extend down to 35 kilometres on average. The continental crust in the Himalayas is 75 kilometres deep.
The mantle is the layer beneath the crust which extends about half way to the centre. It's made of solid rock but can flow over very long periods of time. The convection of heat from the centre of the Earth is what ultimately drives the movement of the tectonic plates and cause mountains to rise. Click here for more details
The outer core is the layer beneath the mantle. It is made of iron and nickel. Complex convection currents give rise to a dynamo effect which is responsible for the Earth's magnetic field.
The inner core is the bit in the middle! It is made of iron and nickel. Temperatures in the core are thought to be in the region of 5000-6000°c and it's solid due to the massive pressure.